1. Field of Invention
Aggregates, traditionally made from natural and crushed stone and sand, constitute about 80% by volume of concrete in structural materials and road construction. The consumption of natural aggregates in the United States is well over one billion tons per year. This invention relates to a process for producing manufactured aggregates from coal combustion by-product. The manufactured aggregates meet standard commercial specifications for road, lightweight and concrete aggregates, making them suitable for use in construction and agricultural applications. The replacement of natural aggregates with aggregates manufactured from coal combustion by-product could provide an opportunity for the high-volume use of and substantially expand the market for coal combustion byproduct.
Coal combustion by-product can come from a number of sources. In response to environmental concerns and regulations, many coal-fired power plants are equipped with flue gas desulfurization ("FGD") technology, particularly wet FGD systems. About 40 million tons of FGD coal combustion by-product were produced by U.S. coal-fired generating plants in 1995 and this amount could increase by an additional 40 to 70 million tons after implementation in the year 2000 of Phase 2 of the Clean Air Act Amendments of 1990. Coal combustion by-products are also produced by fluidized-bed combustion ("FBC") technology and by the thermal treatment of limestone in a coal-fired kiln to produce lime and a lime kiln dust ("LKD") by-product. The typical disposal of FGD, FBC and LKD by-product in landfills impacts the entire nation in terms of cost and land use. Utilization of these by-products as a primary component in manufactured aggregate would reduce waste disposal costs and eliminate the long-term environment liabilities presently associated with disposal.
2. Summary of Prior Art
There are a number of processes in the prior art for making manufactured aggregates or pelletizing coal combustion by-product. The prior art does not, however, teach a process that provides the significant economic and product quality advantages provided by this invention, which results in part from an innovative curing step where the required heat for the curing of the aggregate is generated by the components within the curing vessel, and where fines are converted to a hydrated lime-containing material, which is then used as a component of the aggregate feed mix to improve the strength and wear resistance of the aggregate.
U.S. Pat. No. 4,770,831 issued Sep. 13, 1988 for Process for Manufacturing a Lightweight Aggregate--Walker discloses a process for manufacturing lightweight aggregate by pelletizing or granulating a mixture of pulverized coal fly ash and lime, and curing the pellets at a temperature within the range of from 35.degree. C. to 100.degree. C. in an atmosphere saturated with water vapor from an external source. No curing fines are employed.
A process is described in U.S. Pat. No. 4,880,582 issued Nov. 14, 1989 for Hardening Granulated Material at Elevated Temperatures--Spanjer et al. in which granules are made by pelletizing a mix of p. c. fly ash, lime, water and other components and by embedding the pelletized products with an inert fine-grained material (e.g., fly ash) in a curing vessel at a temperature between 85.degree. F. and 212.degree. F. in a moist atmosphere for about 16 to 18 hours. The required heat is provided by steam injection.
U.S. Pat. No. 5,124,104 issued Jun. 23, 1992 for Coal Pond Fines Agglomeration Holley and U.S. Pat. No. 5,173,232 issued Dec. 22, 1992 for Process for Agglomerating Stack Gas Desulfurization Residue--Holley both disclose coating pellets made from FGD materials with fly ash during one stage of pellet formation by disk pelletizing to facilitate handling and transportation. Unlike the present invention, the patents do not involve aggregate manufacture, curing at elevated temperatures or chemical alteration of the pellets or the use of curing fines heat generation. Construction-grade aggregates can not be produced in these disclosures.
U.S. Pat. No. 5,137,753 issued Aug. 11, 1992 for Pelletizing Ash--Bland et al. discloses pelletizing calcium oxide-containing FBC fly ash after mixing with water and curing the pelletized products by coating with a separately prepared, non-reactive hydrated FBC bed ash to create a low strength pellet for easy disposal. The disclosure is limited to FBC material only. Hydrated FBC bed ash is produced by mixing FBC bed ash with water and converting a substantial portion of the calcium oxide in the FBC bed ash to hydrated lime. The reactivity of the FBC bed ash with water varies, however, with its size distribution. This characteristic can result in an excessive moisture content after hydration and induce bridging problems or uncontrolled agglomeration during curing, which caused the failure of the process when demonstrated at the U.S. Generating Cedar Bay FBC Facility in Jacksonville, Fla. in 1995. In the present invention, manufactured aggregates are produced from a variety of sulfur-containing materials, including wet and dry FGD and FBC materials. Dry fines consisting of p. c. fly ash and fine quick lime are used as the curing medium. The fines remain dry with a moisture content of 5% or less, eliminating the potential bridging problem associated with excessive moisture content and keeping the materials free flowing.
U.S. Pat. No. 5,350,549 issued Sep. 27, 1994 for Synthetic Aggregate Compositions Derived From Spent Bed Materials From Fluidized Bed Combustion and Fly Ash--Boyle discloses a process for making lightweight aggregates from a blend of FBC bed ash and pulverized coal (p.c.) fly ash. The blend is mixed with water and then formed into a shaped article, such as a pellet or a brick, that is later crushed. The shaped articles are cured at ambient temperature while saturated with water for several days.
U.S. Pat. No. 5,364,572 issued Nov. 15, 1994 for Process for Making High-Strength Synthetic Aggregates--Wu et al. discloses a process for making construction-grade aggregates including hydrating calcium oxide-containing coal combustion by-product (e. g., FBC by-product) for a sufficient period of time (5-25 minutes) to convert at least 80% of the calcium oxide to calcium hydroxide prior to forming pellets and curing the pellets at elevated temperature and high humidity with steam addition. Wu does not use curing fines or provide for autogenic heating.
U.S. Pat. No. 4,490,178 issued Dec. 25, 1984 for Granulated Material Loggers et al. discloses a process for forming granulates by simultaneously mixing water, calcium oxide, and fly ash at elevated temperature. The present invention is distinctly different in that recycle fines containing calcium hydroxide, combustion by-product containing sulfur, aluminum-containing material, and water are blended to from a feed mix. The feed mix then is agglomerated and cured to produce a manufactured product.
U.S. Pat. No. 4,064,212 issued Dec. 20, 1977 for Method of Making Pellets Usable as Aggregate or Filler--Kleeberg et al. discloses a process for making aggregate by pelletizing a mix of FGD sludge, fly ash, and a binder, and drying the pelletized products for aggregate production. The water from the sludge is decanted to reduce the moisture content between 20% and 40%, and the sludge moisture content is reduced between 10% to 20% by heating the concentrated sludge before mixing. No curing fines or embedding material is employed in the curing step. External heating, provided either by a rotary dryer or by the hot flue gas, is used to dry the pelletized products for aggregate production. There are several distinct differences between Kleeberg's patent and the present invention. In the present invention, water content of FGD sludge is not limited to 40% or below. The curing step in the present invention is a chemical reaction process driven by the heat generated autogenically by the in situ hydration of CaO, which is added as a component of curing fines prior to the curing step. Autogenic curing with curing fines has the unexpected benefit of providing a uniform heat release and temperature throughout the curing vessel, which results in the production of aggregates of superior physical characteristics for use in construction.
U.S. Pat. No. 5,256,197 issued Oct. 26, 1993 for Abrasion-Resistant Synthetic Article and Method of Making--Smith discloses a process for making solid article or aggregate by mixing fly ash, lime, water, and FGD sludge, and compressing the mixed products at pressures exceeding 1,000 psi, preferably 4000-8000 psi. The compressed products are cured for at least the equivalent of 28 days at 73.degree. F., crushed and sized to produce aggregates. The present invention is distinct because it operates at ambient pressure and uses autogenic heating to cure the product.
U.S. Pat. No. 4,343,751 issued Aug. 10, 1982 for Clay Agglomeration Process Kumar discloses a process in which clay fines alone or in combination with other materials (e.g. fly ash) are agglomerated, dried, and screened for production of end products. Pellets are formed in intensive mixer agglomerators. External heating, provided by Turbo-stack drying systems, is used to reduce pellet moisture content and the tendency of the pellets to coalesce. No curing fines or embedded material is employed in the drying step in this patent.